Many speakers are labeled “constant directivity” because the term seems have become cool in recent years. True constant directivity (I’ll use the abbreviation CD from now on), however, would mean that the directivity of the speaker does not change from the lowest frequency all the way to the highest frequency. It would also mean that the directivity is constant in the horizontal and vertical planes. The only true constant directivity speaker would be a full range, point source hemispherically radiating speaker, something that does not exist in reality.

So what we have left are quasi-CD speakers. We’ll look at the two way designs that have become popular in home theater recently in this article and look at other design approaches such as Constant Bandwidth Transducers (CBTs) and dipoles in future articles.

Constant directivity speakers using waveguides

The design of these speakers takes the form of a woofer with a horn loaded tweeter. The tweeter is generally a compression driver, but does not have to be. There are a few different types of horns, but the one we are interested in is the constant directivity horn. This type of horn is actually often called a waveguide and is just a larger version of those found on forward firing cone / dome speakers. Geddes has written a nice paper that explains what a waveguide is and how it differs in theory from a horn.

The larger the waveguide the lower in frequency the directional control extends to. These speakers are often two ways with a relatively large woofer. The woofer is run up much higher than it would be in a cone/dome speaker and is purposely used to the point at which its response beams. The narrowed response is then matched to the controlled directivity provided by the waveguided compression driver. The theoretical directivity design model for this kind of speaker is wide directivity at low frequencies that narrows as the woofer starts to beam and then stays constant throughout the rest of the frequency range.

These types of speakers have found their niche in high performance home theater. The compression driver used in the waveguide provides very high sensitivity and therefore does not need to dissipate a lot of power to hit cinema SPLs. For Acoustic Frontiers high performance home theater at this point equals constant directivity compression driver based speakers. That really narrows down the list of manufacturers! Companies that design speakers like this are Pro Audio Technology (formerly Professional Home Cinema), Procella (the brand we love and recommend) and the JTR Noesis line. If you want to DIY check out DIYSoundGroup, who provide flat packs and SEOS CD horns. Seaton speakers do use compression drivers but they don’t use true waveguides (they fit into the coaxial driver category).

With these types of speakers the larger the woofer, the larger the waveguide and the lower down in frequency directional control extends. Another example of a well executed CD speaker is JBL’s M2 speaker featuring their strangely shaped Image Control Waveguide. Mix Magazine published an article about the story behind the M2 that is worth reading.

We published this graph earlier, but here it is again: the JBL M2 off axis measurements. You can see by the shape of the off axis curves that the crossover to the waveguide is around 800Hz. Above this frequency the directivity, and therefore off axis spectral balance, is constant through 8kHz or so.

CD waveguided speakers, like the JBL M2 and Procella P8, have some important implications for room acoustics. The off axis sound, at least in the range the waveguide is working, has similar spectral content but it lower in level than the direct sound. Reflections from major boundaries in the room are therefore lower in level than they would be with a wide directivity speaker such as a cone/dome speaker and have fewer perceptual effects. Below the waveguide transition frequency the spectral content of the reflected sound will have a different composition to the direct sound, which may cause unwanted effects. However Floyd Toole’s research shows that image shift, source broadening and timbral coloration from reflections are primarily related to frequencies above 1kHz so maybe the CD design is solid if large enough to provide control down to a low enough frequency.

We would expect the frequency response at the listening position to exhibit a slope from bass frequencies to the point at which the waveguide takes over at which point it should flatten out. This kind of frequency response will look very different to that from a cone / dome speaker. Automated room correction routines that attempt to correct to an arbitrary target curve do not take differing speaker directivities into account, and this is a good reason not to use them unless you can manually draw the target curve. If you find one that lets you set the target curve, such as Dirac Live, a good approach is to measure the uncorrected speakers, use a high level of smoothing such as one octave to show the shape of the frequency response at the listening position, and then fit the room correction target curve to this.

One negative of CD speakers is coverage. Some speakers, such as the JTR Noesis are using very narrow coverage waveguides, with only 60 degrees coverage to the -6dB point (i.e. 30 degrees laterally off axis). When used as screen channel speakers in a home theater this means that at minimum the speakers need to be toed in to provide consistent mid / high frequencies across a row of seats in a home theater. In some wider home theaters the speakers may not have wide enough coverage. When used as surround speakers each row generally needs its own speaker due to narrow coverage. This limitation can be worked around, but needs proper coverage analysis during theater layout. Other speakers use much wider dispersion waveguides, such as the apparent 120 degrees in the M2, or the 90 degrees in the Procella P6.

In future articles we’ll look at other speaker designs such as coaxials, dipoles and CBTs!

As always, thanks for the great info, Nyal. I was looking at the JBL Studio 530’s. They are relatively inefficient (with my AVR I will only be able to pull off a 75db reference level), but they do have a constant directivity horn. I would be using these as right, center, left, and surrounds in a 5.2 setup with my Yamaha receiver. Do you know of the waveguide used on these speakers? Do you know if it has wide or narrow dispersion? Thanks for your time.

I was greatly interested in your article that you wrote about room treatment with control directivity monitors. In particular, I am more interested in what needs to be done at first reflection and second reflection points. If you look at the convention, most acousticians are still saying to use 2 inch or 3 inch absorbers. For a good CD design, how about the Earl Geddes NA12. The directivity sonogram is available on his website and I will provide you a link. In addition I have played with polar map and noticed that if you orient the speakers with a reasonable amount of toe in, and noticed that between 15 to 30° the speaker is actually exceptional in it’s response. However beyond that the speakers power response is also quite good and so that really questions one on whether any treatment is required on the first reflection point. Or if something like what you provide which is a diffuser/absorber would be a more reasonable and smarter option.

Too much absorption, can render a very dead space…but I am confused regarding what Floyd recommends versus what others recommend regarding VER’s.

Let me know your thoughts or arguments in this area. I am very interested in your Vicoustic hybrid products and even the BAD panels, but I do not know if they would apply here as 1st reflection treatment with speakers that are a narrow CD design.

Let’s define first reflections…these are the single bounce reflections from the:

immediate lateral side wall

opposite lateral side wall

ceiling

floor

front wall, behind speakers

back wall, behind listening position

CD speakers are never fully constant directivity; it’s typically only the waveguided loaded driver that could be called constant directivity. Directionality kicks in above a certain frequency, where the wavelengths become small relative to the driver. What frequency that happens at depends on the size of the driver. A 2″ driver will become directional sooner than a 0.5″ driver. Below the frequency where the waveguide is operating the speaker behaves more like a normal cone/dome speaker. You can see these things in the polar plots.

You can set up a CD design in one of two ways with respect to the immediate lateral side wall reflections. You can point them straight ahead, so that the frequency response coming off the side wall is similar to the direct sound, or you can toe them in aggressively, so the high frequencies in the reflection are rolled off relative to the direct sound. The psychoacoustic effect will be different in each situation. A good reference for further reading is the article I wrote on speaker toe-in. With a CD the side wall reflection with toe in has significantly less high frequency energy than a conventional cone / dome speaker. This results in less imaging / soundstaging shifts than you get with the side wall reflection from a cone/dome. However you do have some timbral distortion, because the spectral response of the reflection with a CD is dissimilar to the on axis. Typically you’ll hear less high frequency energy (a darker sonic signature) with a CD than a cone/dome. You can ameliorate this somewhat by using hybrid absorption / diffusion products at immediate lateral side wall reflection points. These absorb the lower frequencies and reflect the high frequencies. Look at the absorption curves of a RPG BAD panel for example, and imagine that if you use one of those at the immediate lateral reflection point for a CD you’ll subjectively increase the high frequency energy in the room.

Hope that provides some insight and please feel free to post any follow up questions that you have!

1. If a CD is used with little to no toe in then the off axis will more closely resemble the on axis spectrally. The impact of the reflection then depends on the off axis angle the reflection hits the side wall and the distance from the speaker to the side wall. You could leave the reflection untreated if you like a broad, spacious sound.

2. Generally I prefer not to use phase based diffusers (with the exception of polys i.e. curved surfaces) for reflections with short path length differences. Amplitude based diffusers are ok, and most of these are actually combination absorber / diffusers.

I think my favorite with CDs is using a 1D, slat based, binary sequence combination absorber diffuser and moderate toe in (not pointed direct at listening position, maybe halfway between firing straight ahead and fully toed in). The Listen Audio diffusers / planks are the ones I am using at present, over a “full spectrum” (absorption down to 100Hz or lower) absorptive backing. Feel free to get in touch if you need further design consulting on the acoustic treatment design of your space.

While it is true that no speaker can be CD over its entire bandwidth, it is pertinent to ask where is CD the most important? Given the fact that at LFs our ability to spatially locate sound diminishes to almost zero (low enough). In a small room it can be shown that localization capability will begin to diminish at somewhere below about 500-700 Hz. – the ear just cannot resolve an impulse in time sufficiently because of the long filter lengths of the auditory filters at LFs. There are a multitude of reflections arriving at each ear within this filter length for frequencies below about 500 Hz. Thus, while CD is critical above say 500-700 Hz it is of dubious importance below that frequency. Happily where we cannot have CD, we don’t really need it.

I agree! Thanks for reading and leaving a comment. The only thing I might mention is that there are very few “CD” 2 way waveguided speakers on the market (your Summa being one of them, the JBL M2 another) that are using crossover points that low…a lot are in the mid-1kHz regions.